The present invention relates to a control apparatus for electric motors, such as direct current motors, which drive electric vehicles, such as golf carts, fork lift trucks and the like, but may be used for any electric drive.
The prior art reveals several attempts to provide control circuits for controlling the current supply for direct current motors in electrical vehicles. For example, U.S. Pat. No. 3,349,309 discloses a variable time ratio control circuit, particularly for use in electrical vehicles, such as small tractors and fork lift trucks, in which power is supplied intermittently to the motor through power switches including a relatively large number of power switching transistors connected in parallel. A feedback circuit including a resistor in series with the motor automatically detects and controls the amplitude of the motor current during each power pulse.
Such a circuit, however, has disadvantages which arise in the case of breakdown of one or more of the power switching transistors. In the circuit, the total current flowing through the power switching transistors is detected, and is controlled so that its value may not exceed the maximum allowable value. However, supposing that one of the power switching transistors connected in parallel has short-circuited and broken down for any cause, and thereafter has electrically opened. Then, current which was previously carried by the disabled transistor flows into the other power switching transistors resulting in an increase in the current thereof. As the number of disabled transistors increases, the amount of current flowing through each of the remaining transistors increases, and finally, the current level reaches a value where all the power switching transistors break down at once.
Another prior art system which is relevant to the present invention is disclosed in U.S. Pat. No. 3,803,471, filed on Apr. 9, 1974, and directed to "Variable Time Ratio Control Having Power Switch Which does not Require Current Equalizing Means." However, this system also has disadvantages in its operation.